The little boy was diagnosed at five months old with severe combined immunodeficiency disorder (SCID), sometimes called "bubble boy disease," and has spent more than half of his life in a sterile environment (a "bubble").

The story spread to every corner of the globe, and today thousands of people are wearing the sunny hue for the boy from Northamptonshire to raise awareness for his disease and remind him he's not alone.

Seth's dad, Nik, said earlier that they have a map in his son's hospital room where they're going to pin all the countries in the world that people have shared a picture from, and post some of the pictures on the walls.

A band even wrote a song for Seth with the lyric, "We're all wearing yellow for you."

Sometime next week Seth is scheduled to have his gallbladder removed, due to taking antibiotics for years. If that's successful, he'll start chemotherapy in five weeks, then eight days later have his second bone marrow transplant.

Leanne said that after the transplant, Seth will likely be in the hospital for up to another year, but said he "deals with everything extremely well."

"When we went to the hospital, he said to me, 'I'm not going home for a long time Mummy, am I?' I said, 'No, you've got a lot to do here' and he comes to the conclusion that he's just accepted it," she told ABC News.